Easiest quickest way to lower ph?

I tried searching but ph is too short to search for.

What's the easiest and quickest way to lower the soil ph? One of my plants appears to be showing signs of a zinc defficiency and I'm almost 100% sure it's because of high soil ph... not that I have a meter or papers. Is there any household substance I can use to lower the ph, I'd get some ph down but it could take a few days to get to the store. I seem to remember something about using baking soda, but for all I know I'd raise the ph and kill the plant.

 

any idea where i'd get a kit and the cost? i don't have any hydro stores around here and i don't know of any chemistry stores that are in the area. i may be able to get someone to swipe some ph paper from a hs science class but so far no one knows about my grow yet.